This application relates to a process for the production of highly-coercive isometric magnetic iron oxide pigments and to magnetic iron oxide pigments produced by this process and to the use thereof.
Magnetic iron oxide pigments of the composition FeO.sub.x, wherein 1.33.ltoreq.x.ltoreq.1.5, are used for the production of magnetic toners for electrostatic photo-copying processes or for magnetic printers. Isometric magnetite pigments having coercive field strengths (I.sup.H c) of from about 80 to 130 Oe are substantially used in conventional processes for this purpose. However, greater demands are made on these pigments in modern one-component toners. The coercive field strengths thereof have therefore to be within the range of from 180 to 250 Oe or above.
Two processes are essentially known at the present time for obtaining the coercive forces required. Those are:
1. by the incorporation of cobalt into the iron oxide pigments, PA0 2. by the use of needle-shaped pigments. PA0 (a) Fe.sub.3 O.sub.4 pigments produced in a wet process are oxidized and PA0 (b) are subsequently reduced to produce relatively highly-coercive Fe.sub.3 O.sub.4 pigments. PA0 (c) oxidized in a further step to produce relatively highly-coercive .gamma.-Fe.sub.2 O.sub.3 pigments or bertholloid mixed phase pigments.
However, the incorporation of cobalt is expensive and
in addition causes an undesirable fall in the I.sup.H c values as the temperature increases. Needle-shaped pigments produce directional dependence in the toner and rheological problems in the production thereof with the result that their use is accompanied by disadvantages.